It's a "state container" because it holds all the state of your application. It doesn't let you change that state directly, but instead forces you to describe changes as plain objects called "actions". Actions can be recorded and replayed later, so this makes state management predictable. With the same actions in the same order, you're going to end up in the same state.

My answer can be silly, but Redux concepts help to explain and understanding the meaning of predictable in this context. That means that you always know from where the state comes from, (from the store), who will be able to update it, (Actions).